Awesome
ecma262#3195
This is the repo for the nomative change of https://github.com/tc39/ecma262/pull/3195.
Status: Stage 2
Champions: Chengzhong Wu (@legendecas)
Motivation
The goal is avoid revealing internal slot [[PromiseState]] with
promise.then(onFulfill, onRejection)
to the promise handlers by
host hook requirement.
ECMA262 HostEnqueuePromiseJob defines that a host must implement it as:
- Let scriptOrModule be GetActiveScriptOrModule() at the time HostEnqueuePromiseJob is invoked. If realm is not null, each time job is invoked the implementation must perform implementation-defined steps such that scriptOrModule is the active script or module at the time of job's invocation.
The timing of HostEnqueuePromiseJob
depends on internal Promise
[[PromiseState]]
. The motivation instead expect the ActiveScriptOrModule
been determined AT the time promise.then()
is invoked.
// module A
const {promise, resolve} = Promise.withResolvers();
promise.then(handler); // (1)
// module B
resolve(); // HostEnqueuPromiseJob for (1)
promise.then(handler); // (2) HostEnqueuPromiseJob
That is, in the above example, the ActiveScriptOrModule for the handler at call site (1) should be "module A", and at call site (2) should be "module B".
HTML integration
HTML HostEnqueuePromiseJob doesn't follow the requirements of ecma262.
However, HTML spec defines that the active ScriptOrModule is saved at the time of HostMakeJobCallback is invoked rather than at the time of HostEnqueuePromiseJob is invoked. The two host hooks are invoked with the same active script or module consecutively.
This change also mandates that the active scripts are propagated through promise jobs and FinalizationRegistry cleanup callbacks.
For example, the following example uses HTML APIs that depends on the original script's incumbent document URL.
const frame = frames[0];
const setLocationHref = Object
.getOwnPropertyDescriptor(frame.location, "href")
.set
.bind(frame.location);
framePromise.resolve("./page-1").then(setLocationHref);
This proposed behavior exists in the HTML spec since HTML
HostMakeJobCallback
already propagates the active script.
Proposed change
Checkout https://github.com/tc39/ecma262/pull/3195
The reality of the implementation status is:
. | resolved.then(f) | resolve.then(v => f(v)) | pending.then(f) | pending.then(v => f(v)) |
---|---|---|---|---|
HTML | outer | outer | outer | outer |
ECMA-262 | outer | outer | ☹️ inner | ☹️ inner |
Chrome/Firefox | ☹️ inner | outer | ☹️ inner | outer |
Safari | outer | outer | ☹️ inner | ☹️ inner |
- pending is resolve in inner iframe.
- f is iframe
setLocationHref
.- outer: url is resolved to outer page.
- inner: url is resolved to inner iframe.
Conclusion:
- ECMA-262 and Safari reveal the
[[PromiseState]]
. - Chrome/Firefox don't respect
.then(f)
equals to.then(v => f(v))
- HTML and the proposed behavior have neither of those problems
The proposed change also fixes the inconsistent requirement on ecma262
HostEnqueuePromiseJob
and HTML HostMakeJobCallback
.